block heater

One way to do a simple test without tools is to unplug it quickly in a dark setting and see if there is a spark. That will show if it is drawing power.

Also, are you saying that it started harder when using the block heater than it did in the past without using it? Sometimes the block heater puts enough warmth in the engine that it won't call for the glow plugs to cycle.

Yep. How many times have I seen extension cords stretched across the parking lot? LOL. They do quit, the element gets covered with deposits from the cooling system or simply burns out. Is yours in the block or a heater hose? The latter will airlock and the heater fails.

Check for continuity between the prongs of the cord.

how do i check the cord or heater to see which is bad?

The most important thing to remember with a block heater is to unplug it before you drive off. I keep the extension cord over my side mirror as a reminder...

Yep, it may have died.
FWIW, a long time diesel mechanic told me that starting the eng with the heater running seems to kill them (not always).
The one I helped a guy replace was subjected to that treatment on a regular basis.
Not trying to get into a argument here. Just pointing out a trend noticed by a top notch mechanic.

Get a Kill-a-watt meter. Use that to check to see if the element is drawing power.

When the power to the block heater first kicks on, can you listen carefully near the radiator and hear a popping or sizzling noise indicating that the heater is actually warming up?

Was the engine warmer than not plugged in?
Block heater afaik is not tied into the operating systems. It either works or it doesn't.
But gotta be more specific if looking for recommendations.